Wave transmission branching arrangement



July 31, 1956 J. cs. LlNVlLL 2,757,342 WAVE TRANSMISSION BRANCHING ARRANGEMENT Filed March 3, 1955 MEGA 771 5 IMPEDANCE F/L 7'5? 1. 0,40 CONVERTER 2 l I A VVY NEGA T/VE 5 IMPEDANCE FILTER 1.0,40

CONVER TB? /N l E N TOP J. G. L //V V IL L A TTOR/VEV portion of the transmission United States Patent 0 f f I ARRANGEMENT J Jul Inc; Linvill Ii, assignor to Bell Tele= phone Laboratories, Incorporated, New YorlgN. Y.,. a corporation of New York 'Ilhisinventionrelates: to wave=transmissionsystems and more particularly toabranching.arrangemen't:for aamultich'annelsystem.

The object of the inventionis to-increaseithe-discrimis nation between channels in a multichannel? wave trans mission system, Related objects-are .to-decrease the cbmplexity and-reduce 'the-castof the wave filtersrequired ati a br'anchingpoint inasuch a system.

In carrier telephone and other wave tr ansmissionsyis terms it is necessary at certain points.- to se'parate a-multifrequency signal into individual channels-entire basis of frequency. This may be done by connecting the channelsto the branching point in parallel and providing. inthe various channelsfilters: whichhave nmtuallyexclusive p'assbands. If these pass bandsare lo'catedclosetogether; the filter structures must be quite complex inrorder to. prosvide the required discrimination between channels.

; Inthe branchingtarrangement of the present invention; thediscrimination between channels is greatly increased; or, alternatively, the complexity and.cost-ofithefilters'are' reduced, by adding a negative impedance converter and a resistor to each branchlnr'thedllu'strative embodiment shown, each branch comprises a bandpass wave filter, a negative imped'ance converter connected in tandem between the filter and the branching point, and a series resistance connected between the converter and the branching point. The filter has an image impedance which is resistive and preferably uniform 'over the major band. The converter has an impedance conversion ratio so chosen that the impedance of the filter in the band, as seen through the converter, is approximately equal to the negative of the associated series resistance. The series resistances in .the various branches are preferably, but not necessarily, approximately equal. In order to stabilize the operation of the converters, the impedance of the multifrequency signal source must be resistive in the pass bands of the filters, and is preferably approximately equal to the added series resistance in one of the branches.

The combination of the negative resistance presented by the converter and the associated series resistance provides at the branching point a substantially zero impedance, over the pass band, looking into the appropriate channel. This greatly increases the discrimination between channel's obtainable with given passive filter structures, 'or provides the same discrimination with simpler and less expensive structures.

The nature of the invention and its various objects, features and advantages will appear more fully in the following detailed descrip'tion of a typical embodiment illustrated in the accompanying drawing, the single figure of which is a schema-tic diagram of a two-channel branching arrangement in accordance with the invention.

A multifrequency source of alternating current signals 1 is shown connected to a pair of terminals 2 and G which constitute a branching point. The source preferably has a resistive impedance, as indicated by the series resistance of value Rs. At the point 2-'3 the signals are separated,

Patented July 31,1956;

2 on the basis. of frequency, intothe two transmission branches 4 and 5,. which-are terminated; in the matching loads 6 and 7. Between the load fi and the point. 2- 3,. the branch 4 comprises, i-n-the order named, at bandp asswave' filter '4 -a negative impedance converter '9 connected in; tandem therewith; and &' series resistor of value R1. A filter=11 0,.'a negative impedance converter 111-, and a series resistor of value R2 'aresimil-arl-y 'disposed inthe branch 5. The resistances R1 and R2 are' prefe'rably, but not. necessarily, equal. It is to be understood that any required number of additional-dbranchesmay be connected at the points The: filters 8- and-1d have mutually exclusive transmission bands. Each has an image impedance which is resistive and preferably sub stantially constant over at least m'ost'vof they pass band,- and is a comparatively high. reactance outside ofthis band; It the image impedance is not constant over theband,; it ispreferablethat the impedanceafrequency charac'teristic be concave downwarcht that is, that it has its maximum value atthe mid-band frequency and falls to: zero at the band limits. A-suitablecharacteristieis obtainable,,- for exagrnple with. a constant-k:- type of filterwith: a mid-series termination. A concave-downward type of: characteristic resultstinmore stable operation. of the converter than if-. "aconcave-upward characteristic is Y, ploye'dg The image impedance of the filter at midband is preferably approximately equal to the associated-resist anee Rnor-Rzs I The converters 9 and 1 0 may, for example-bowl" the type disclosed in Patent 2,726,370, tO'R.L. Wallace, In and the present applic'antaissued December 6 ,-,1:95'5.- Each converter has an. impedance conversion ratiosuch thati the impedance at the midband frequency ofthe associe. ated filteras seen'through. the-convertends approximately equaL-to thenegative ofthe associated seriesresistan'ce R1 or R22; Thus,-rif theimagedmpe'dance in the band ofi the; filter; "8:.is substantially constant and. approximately equal to R1, the converter 9 is designed to have an impedance conversion ratio of 1. Of course, the filter 8 may have an image impedance at midband which is either larger or smaller than R1, in which case the converter '9 will have the appropriate inverse impedance conversion ratio. If the filter 8 has 'a concave-downward characteristic in the band, it will be advantageous to make its value at midband slightly greater than R1. Then, the impedance seen at the left looking into the converter 9, with an impedance conversion ratio of 1, will be equal to R1 at two frequencies, one on either side of midband, instead of only at the midband frequency.

It is thus apparent that, in the pass band of the filter 8, the branch 4 presents at the branching point 2-3 an impedance which is approximately equal to R1R1, and is very nearly zero. Similarly, in the band of the filter 10, the branch '5 has an input impedance approximately equal to Rz Rz, which is also nearly zero. Outside of the pass band, each of the branches 4 and 5 has an impedance which is high compared to the impedance at midband. Therefore, signal currents from the source 1 in the frequency range of the filter 8, when they arrive at the branching point 2 3, select the low-impedance shunt path presented by the branch 4, and very little of this energy reaches the branch '5. Similarly, substantially all of the signal currents falling in the pass band of the filter 10 enter the branch 5 in preference to the branch 4. Thus, the discrimination between channels provided by the filters 8 and 10 is greatly augmented by that due to the addition of the converters 9 and 11 and the resistors R1 and R2. In a two-channel branching arrangement which has been successfully operated, this latter factor improved the discrimination by about 30 decibels.

In order to stabilize the operation of the converters 9 and 11, the signal source -'1 should present a resistive impedance, shown as Rs, to the branching point 2-3. The resistance *Rs is preferably approximately equal to R1 or R2 when the latter are approximately equal, or to the average of R1 and R2 if they differ considerably. It the internal resistance of the source -1is less than the desired value -Rs,-it may be built out by'the addition of an external series resistance of proper magnitude.

t 'It is to be understood that the above-described arrangement is illustrative of the application of the principles of the-invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.-

What is claimed is e i 1. In a wave transmission system, a branching point and a plurality of transmission branches connected in parallel at said point,- each of said branches'comprisin'g a wave filter having a resistive image impedance in the transmission band, a negative impedance converter connected between said filter and said point, and a resistor connected in series between said converter and said point, said converter having an -'mpedance conversion ratio so chosen that the impedance of said filter as viewed through said converter is approximately equal to the negative of the resistance of said resistor at a frequency in said band.

2.- A system in accordance with claim 1 in which the resistors in said branches are approximately equal in resistance.

3. A system in accordance with claiml in which the resistance of said resistor is approximately equal to said image impedance at the midband *frequency of said band and said ratio is approximately equal to -l.

4. A system in accordance with claim 1 in which said image impedance is substantially constant over the major portion ofsaid band.

5. A system in accordance with claim 1 in which the frequency characteristic of said image impedance is concave downward in said band;

6. A system in accordance with claim 5 in which said ratio is so chosen that the maximum value of the impedance of said filter in said band as seen through said converter is slightly greater than the resistance of said resistor.

' branching point, a source 7. A system in accordance with claim 1 which includes a resistive source of multifrequency signals connected to said branching point.

8. A system in accordance with claim 7 in which the impedance of said source is approximately equal to the resistance of one of said series resistors.

9. A system in accordance with claim 8 in which the resistors in said branches are approximately equal in resistance. I

10. A system in accordance with claim 1 in which the filters in said branches have mutually exclusive transmis sion bands.

11. In a multichannel wave transmission system, a

of multifrequency signals having a resistive impedance Rs connected to said point, "and a plurality of branches connected in parallel at said point, each of said branches comprising a wave filter having an image impedance which is resistive and substantially constant over the major portion of the transmission band, a negative impedance converter connected in tandem between said filter and said point, and a resistor having a resistance approximately equal to Rs connected in series between said converter and said point, said converter having an impedance conversion ratio so chosen that the impedance of said filter as viewed through said converter is approximately equal to Rs at a frequency in said band.-

12. A system in accordance with claim 11 in which said imageimpedance is approximately equal to "Rs at the midbandtfrequency of said filter and said ratio is approximately equal to -l.

'13. A system in accordance with claim ll in which the frequency characteristic of said image impedance is concave downward in said band.

14. A system in accordance with claim 13 in which said ratio is so chosen that the maximum value of the impedance of said filter as viewed through said converter is slightly greater than Rs.

. No references cited. 

